FIG. 19 is a view showing a decompression mechanism 100 according to a prior art. Japanese Laid-Open Patent Application Publication No. 2001-173421 discloses a decompression mechanism 100 which is configured to slightly open an exhaust port to reduce a compressive pressure when compressing an air-fuel mixture in the interior of a cylinder, as a mechanism for reducing a start torque at the start of an engine. The decompression mechanism 100 is provided at an exhaust cam 101, and includes a sleeve 102, a decompression pin 103, and a decompression shaft 104. The sleeve 102 accommodates the decompression pin 103 such that the decompression pin 103 is extendable and retractable. The sleeve 102 is fitted into an accommodating hole 105 of the exhaust cam 101. The decompression shaft 104 causes the decompression pin 103 within the sleeve 102 to be extended and retracted.
The decompression pin 103 has a structure in which a tip end portion 103a protrudes from a cam surface 101a of the exhaust cam 101 at the start of an engine and is configured to contact a locker arm in a compression stroke of the engine. Thereby, an exhaust valve is pressed down in the compression stroke, slightly opening an exhaust port. This reduces a start torque which is required to move the piston up to a top dead center. After start of the engine, the tip end portion 103a of the decompression pin 103 is retracted to be inward relative to the cam surface 101a. For this reason, after the start of the engine, the exhaust valve is not pressed down in the compression stroke and the exhaust port is closed, so that the air-fuel mixture can be compressed with a higher pressure than the pressure at the start of the engine.
In the structure of the publication No. 2001-173421, in which the decompression mechanism 100 is provided at the exhaust cam 101, when the exhaust cam 101 rotates, a centrifugal force which causes the sleeve 102 to come off from the exhaust cam 101 is applied to the sleeve 102. To avoid the sleeve 102 coming off from the exhaust cam 101 due to the centrifugal force, the sleeve 102 needs to be accommodated in an accommodating hole 105 by an interference fit and fixed to the exhaust cam 101. However, since the interference fit causes reduction in a tolerance of the accommodating hole 105 and the sleeve 102, high dimension accuracy is needed. Therefore, forming the exhaust cam 101 and the sleeve 102 is time-consuming work. In addition, incorporating the decompression mechanism 100 in the exhaust cam 103 is also time-consuming work. Such a situation occurs in a valve operating system other than the locker arm valve operating system.